Solid combustible composition containing epoxy resin



United States Patent 3,130,096 SOLID COMBUSTIBLE COMPOSITION CONTAINING EPOXY RESIN Malcolm E. Pruitt, Wallace T. McMichael, and Elmer L. Pendleton, Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Nov. 9, 1961, Ser. No. 151,165 8 Claims. (Cl. 149-19) The invention relates to solid combustible compositions and to charges formed thereof which are adaptable to gas generation at high velocity. The invention more particularly relates to solid propellants for use in rocket propulsion which are capable of high ballistic performance.

In the combustion chamber of a rocket propulsion motor, propellant mixtures are burned to produce gaseous combustion products which are exhausted through a nozzle at high velocity to produce a thrust. Solid propellant charges are commonly composed of a fuel, an inorganic oxidizer such as ammonium perchlorate or ammonium nitrate, and a binder.

The term, propellant, as used herein, refers to a substantially homogeneous solid mixture comprising any of the known oxidizing agents employed in the field of propellants in admixture with a suitable fuel and binder to produce a solid which can be ignited and which thereupon releases essentially gaseous combustion products at high temperature, e.g., above 2000 F. The fuel and binder may be the same or different substances. In order to increase the flame temperature of the propellant and thereby increase the gas expansion ratio, it is common practice to admix known metal fuels, in a particulated state, such as aluminum, magnesium, lithium, beryllium, boron, and alloys thereof and metal hydrides with the fuel-oxidizer-binder mixture.

The term, fuel, as used herein, refers to substances which liberate heat during burning and which act as a chemical reducing agent, i.e., are themselves oxidized by the oxidizing agent.

A large number of fuels, oxidizers, and binders have been used. The fuels, broadly, are materials which can be oxidized sufilciently rapidly in the combustion chamber of a rocket engine to release gases to provide the desired thrust but are not violently explosive under the prevailing conditions. They may be solid or liquid. Solid fuels commonly employed contain combined hydrogen and/ or combined or elemental carbon in a combustible state. Known fuels include pulverized coal, bitumens, petroleum residium, asphalts, cellulose, rubbers, and a large number of derivatives thereof including metals chemically combined with organic radicals containing hydrogen and/ or carbon. The finely ground metals named above are also fuels but are usually considered as modifiers and are generally employed with another fuel of the hydrogenor carbon-containing type.

Known binders include certain natural and synthetic resins, e.g., butadiene-styrene and butadiene-acrylate rubbers, polyesters, phenol-formaldehyde resins, ureaformaldehyde resins, polyacrylates, polyalkylacrylates, l. polystyrene, polysulfides, polyurethanes, and polyvinyl acetate resins.

The solid propellants may be cast into predesigned geometrical shapes of convenient size, e.g., tubular, cylindrical, triform, hexaform, and a variety of modifications ice thereof including those having multiperforations therein or which may be cast directly in the combustion chamber of the rocket motor.

Solid propellant compositions employing known binders, however, are not completely satisfactory and present problems which have not yet been solved. Many of the known binders require lengthy and complex processing to produce required characteristics for satisfactory use in solid propellants. The introduction of extraneous and undesirable impurities is often necessary in preparing the propellant changes. For example, curing agents and plas ticizers are often required. The curing agents and plasticizers frequently interfere with uniform combustion, give off toxic gases, and leave residues in the combustion chamber. Furthermore, the use of plasticizers to prevent freezing of the solid propellant mass at low temperature, e.g., in the range of from 20 to F. has not been fully satisfactory. Freezing is to be prevented because it causes brittleness which often results in fissures in the charges which increases the burning surface and consequently adversely affects the rate of gas evolution. An undesirable consequence of such fissures is a constricting effect on the exhaust nozzle, thereby causing an inadequate passageway therethrough and often resulting in high internal chamber pressures and sometimes damaging explosions.

It is, therefore, an object of the invention to provide an improved solid propellant composition of superior physical properties. It is a further object of the invention to provide a solid propellant having a high specific impulse and particularly adapted for use in rocket motors. How these and other objects are attained will become apparent as the ensuing description proceeds.

The invention is predicated on the discovery that a mixture of a diphenolic diglycidyl ether and an aliphatic diglycidyl ether containing a curing agent provides both a fuel and binder when admixed with an oxidizing material, with or without a modifying fuel additive such as a pulverized metal, and produces a solid propellant of superior strength, flexibility, and burning properties. The term curing agent as used herein refers to any of the known curing agents, hardeners, or cross-linking agents for epoxy resins and includes the amines, polyamines, dibasic acids and anhydrides thereof, and Lewis acids and ether and amine complexes thereof.

The composition of the invention may be cast into any desired geometric shape or configuration without the necessity of high pressure molding or extrusion. It offers the further advantage of resiliency and excellent dimensional stability; a tensile strength of not less than 175 pounds per square inch, is capable of significant elongation, ranging from 5 percent to greater than percent, and possesses excellent case-bonding properties. Combustion of the solid propellant of the invention in a combustion chamber proceeds at a substantially uniform rate of from about 0.3 to 0.6 inch per second at 1000 p.s.i. chamber pressure without detonation, appreciable smoke, or residue.

The invention, accordingly, is a novel solid propellant employing (A) a fuel-binder having the composite properties of flexibility, bond strength, and uniform combustibility in admixture with (B) a known inorganic oxidizing agent. It consists of the oxidizing agent in admixture with a mixture of two diglycidyl ethers having the Formulae 1 and 2 set out below:

wherein R is an isoalkylidene residue of l to 4 carbon atoms, wherein n is from to 4 and the resulting epoxide equivalent weight is from about 156 to about 773. (Epoxide equivalent weight means the number of grams of the diglycidyl ether necessary to yield one oxirane group.) This ingredient may be referred to, generally, as an aromatic diglycidyl ether.

CHg-CH-CHg-EO-R'3-O-CH1*CHCH:

wherein R is an alkylene residue of from 2 to 4 carbon atoms, wherein m is from zero to about 90, and the resulting epoxide equivalent weight is between 65 and about 3000. This ingredient is referred to hereinafter, at times,

as an aliphatic diglycidyl ether.

Although the term, diglycidyl ether, is employed herein, it is to be understood that the ethers concerned usually contain somewhat less than an average of two glycidyl groups per molecule. It is to be further understood that the diglycidyl ethers include the polyethers containing repeating ether groups represented by the bracketed portions of Formulae 1 and 2 above.

The preferred aromatic diglycidyl ethers to employ are those prepared by reacting epichlorohydrin with a binuclear diphenol, i.e., a bisphenol, especially an isoalkylidene diphenol of which 4,4'-isopropylidene diphenol known as bisphenol A is illustrative. The preferred aliphatic diglycidyl ethers to employ are those prepared by reacting epichlorohydrin with a polyoxyalkylene glycol, sometimes called merely a polyglycol, of which polyoxypropylene glycol having a molecular weight of between 300 and 500 and more often of about 400 is illustrative. They include such compounds as the homopolymers of a1- kylene oxides having from 2 to 4 carbon atoms terminated at either end by oxirane groups and polyhydroxy initiated polyglycols terminated with oxirane groups.

The weight ratio of the aromatic diglycidyl ether to the aliphatic diglycidyl ether to employ is between 0.02 and 1.5 and the preferred ratio to employ is between 0.1 and 1.0. The total amount of the diglycidyl ethers, including the curing agent therefor, to employ is between and 35 percent of the propellant composition, the balance being oxidizer and a supplemental or modifying metallic fuel, when employed. It is preferred that a supplemental or modifying metallic fuel, e.g., pulverized aluminum metal, in an amount up to about 30 weight percent, and preferably between about 10 and 25 weight of the propellant composition, be employed.

One embodiment of the invention contemplates employing, in addition to the diglycidyl ethers represented above, a triglycidyl ether of glycerol or the triglycidyl ether of a glycerol-propylene oxide adduct in amount up to about one-third of the total glycidyl ether components. Similarly, as in the diglycidyl ethers, the material designated triglycidyl ether usually contains slightly less than an average of three glycidyl groups per molecule.

To prepare the composition of the invention, the diglycidyl ethers, as represented by the above formulae, within the weight ratio stated, and containing in intimate admixture therewith a sufiicient amount of a suitable curing agent to set the resin, are admixed in any suitable type mixer, e.g., one provided with baflles and a rotating shaft equipped with impeller blades.

The admixture so made is promptly cast in suitable shaped charges for subsequent loading into a combustion chamber but is preferably cast directly into the combustion chamber of a rocket motor and there permitted to set to the solid charge. The charge so made, when ignited by known means, e.g., a fulminate detonator or a primer charge of black powder which is set off by a timed electric spark, burns evenly. A steady flow of evolving gases proceeds from the burning propellant charge and escapes at high pressure through the nozzle.

The time required to cure or set the resin fuel-binder of the propellant composition made according to the in- I of the invention but are not to be construed as defining the limits thereof.

Example 1 A blend was prepared which consisted of the following parts by weight.

Ingredient: Weight percent Diglycidyl ether of bisphenol A 1 having an epoxide equivalent weight of from 475 to 575 6.57 Diglycidyl ether of polypropylene glycol having an epoxide equivalent weight of about 330 and a viscosity of 42 cps 13.13 Aluminum powder, through 325 mesh (U.S.

sieve series) 14.78 NH ClO through 250 mesh 19.21 NH CIO unground 44.83 Monoethanolamine 1.258 Diethylenetriamine 0.222

1 Bisphenol A is 4,4-isopropylidenediphenol; it has a Durrans softening point of 74 to C.

The above components were mixed for 30 minutes at F. The resulting composition was a fiowable slurry of relatively high viscosity. It was drawn from the mixer and compressed to a uniform thickness of 0.25 inch and placed in an oven and cured therein for 2 hours at 227 F. The cured sheet of propellant thus made was removed from the oven and cut into strips 6 inches long and 0.75 inch wide. These strips were tested and found to have a tensile strength of 176.8 pounds per square inch, an elongation of 55 percent, and excellent adhesion to glass, metal, and ceramic surfaces. The example was repeated except that the fiowable slurry was cast into specimens for use in determining the burning rate.

The cast composition was tested according to standard testing procedures to ascertain its burning rate which was found to be between 0.28 and 0.34 inch per second at 1000 psi. gauge pressure.

The above example was repeated except no aluminum powder was employed. The composition comprised 78 percent NH ClO oxidizer and 22 percent of the mixture of diglycidyl ethers employed above containing the amine curing agents employed therein. The burning rate was found to be between 0.48 and 0.62 inch per second at 1000 p.s.i. gauge pressure.

Example 2 Another composition illustrative of that of the invention was prepared employing the following components.

Ingredient: Weight percent Diglycidyl ether of bisphenol A having an epoxide equivalent weight of 173 9.735 Diglycidyl ether of polyoxypropylene glycol having an epoxide equivalent weight of 330 9.735 Monoethanolamine 2.517 Diethylenetriamine 0.133 Aluminum powder, 325 mesh .f 14.60 NH ClO 250 mesh 18.98 NH ClO unground 44.30

The procedure employed was substantially the same as that of Example 1. After the 2-hour cure, the sheet of propellant composition was removed from the oven and tested and found to have a tensile strength of 633.2 pounds per square inch, an elongation of 15 percent, and excellent adhesion to metals and ceramics.

Example 3 Another composition illustrative of the invention was prepared by admixing the following ingredients.

Ingredient: Weight percent Diglycidyl ether of bisphenol A having an equivalent weight of 173 7.09 Diglycidyl ether of polyoxypropylene glycol having an epoxide equivalent weight of Monoethanolamine 2.166 Diethylenetriamine 0.114 Aluminum powder, 325 mesh 15.00 NH CLO 250 mesh 19.50 NH CIO unground 45.50

The mixing and curing procedure employed in the Examples 2 and 3 was followed. The sheet of propellant so made was tested and found to have a tensile strength of 500 pounds per square inch, an elongation of 14.5 percent, and excellent adhesion to solid surfaces.

Example 4 To illustrate further the composition of the invention, the following ingredients were admixed.

Ingredient: Weight percent Diglycidyl ether of bisphenol A having an epoxide equivalent weight of 173 2.38 Diglycidyl ether of polyoxypropylene glycol having an epoxide equivalent weight of 330 0.44 Curing agent composed of 85 weight percent aniline and 15 weight percent meta-phylene diamine 3.1 Aluminum powder, 325 mesh 20.0

Oxidizer composed of 70 weight percent NH CIO (unground) and 30 weight percent NH ClO ground to 250 mesh 65.0

The above ingredients were admixed in accordance with the procedure employed in the above examples and cast into sheets as above described. The curing temperalow aluminum dish and there cured in the shape of a flat cake. The curing period employed was extended to 40 hours at 85 C. The product so made was tough and flexible.

Example 6 The following composition was prepared to show the effect of incorporating a polyglycidyl ether containing more than an average of two glycidyl groups per molecule with the composition of the invention.

Ingredient: Weight percent Diglycidyl ether of bisphenol A having an epoxide equivalent weight of 475-575--" 3.53 Diglycidyl ether of bisphenol A having an epoxide equivalent weight of 187-193---- 1.76 Diglycidyl ether of polyoxypropylene glycol having an epoxide equivalent weight of 330 7.92 Triglycidyl ether of a glycerolpropylene oxide adduct having an epoxide equivalent weight of 300-400 4.40 Curing agent consisting of 85% aniline and 15% by weight metaphenylene diamine 2.4 Aluminum powder (passing through a 325 mesh screen) 20.0 Oxidizer consisting of 70% of unground NH ClO and 30% by weight of NH C1O pulverized by grinding in a Mikro-Pulverizer at 9800 r.p.m 60.0

The mixture was mixed for 0.5 hour at 30 C. and then cast into sheets and cured at 85 C. for hours. The sheets were then tested and found to have a tensile strength of 389 psi. and an elongation of 5%.

The castings made in the example above had a theoretical specific impulse of greater than 240 seconds as calculated by standard methods based on thermal values.

Having described our invention, what is claimed and desired to be protected by Letters Patent is:

1. The solid propellant composition which comprises an intimate mixture of (A) between 10 and 35 percent by weight of a fuel-binder consisting essentially of a thermosetting glycidyl ether mixture of (1) an aromatic diglycture employed was the same but the period of cure was 45 idyl ether having the formula extended to 16 hours. The product thus made was a particularly flexible and strong propellant composition having good physical properties.

Example 5 A further composition illustrative of the invention was prepared by admixing the following ingredients.

Ingredient: Weight percent Diglycidyl ether of bisphenol A having an epoxide equivalent weight of 173 6.16

The above ingredients were admixed in accordance with the procedure employed in the above examples except that the liquid composition was poured into a shalwherein R is an isoalkylidene residue containing 1 to 4 carbon atoms, n has a value of 0 to 4, and the epoxide equivalent weight is from about 156 to about 773, and (2) an aliphatic diglycidyl ether having the formula wherein R is an alkylene residue containing from 2 to 4 carbon atoms, m has a value of from 0 to about 90, and the epoxide equivalent weight is between about 65 and about 3000, in a weight ratio of (l) to (2) of between 0.02 and 1.5, and a curing agent for said diglycidyl ethers consisting of a mixture of a major portion of (a) a monofunctional amine and a minor portion of (b) difunctional and polyfunctional amines, in an amount suflicient both to extend the chain growth and to effect cross-linking of the diglycidyl ethers, and (B) from to 65 percent by weight of an inorganic oxidizing agent.

2. The composition of claim 1 wherein the weight ratio of (1) the aromatic diglycidyl ether to (2) the aliphatic diglycidyl ether is between 0.1 and 1.0.

3. The composition of claim 1 wherein said monofunctional amine is an alkanol amine and said difunctional amine is a dialkylenepolyamine.

4. The composition of claim 1 wherein said monofunctional amine in aniline and a said polyfunctional amine is metaphenylene diamine.

5. The composition of claim 1 which contains up to 30 percent by weight, based on the weight of said composition, of a readily oxidizable particulate metal selected from the class consisting of aluminum, magnesium, boron, beryllium, and lithium.

6. The composition of claim 3 wherein the percent by weight of (A) said fuel-binder is between 15 and 30, of (B) said inorganic oxidizing agent is between 45 and 60, and said readily oxidizable particulate metal is between 10 and 30 percent.

7. The composition of claim 6 wherein (A) is a di- References Cited in the file of this patent UNITED STATES PATENTS 3,002,830 Barr Oct. 3, 1961 3,022,149 Cramer Feb. 20, 1962 3,028,271 Dixon et a1 Apr. 3, 1962 

1. THE SOLID PROPELLANT COMPOSITION WHICH COMPRISES AN INTIMATE MIXTURE OF (A) BETWEEN 10 AND 35 PERCENT BY WEIGHT OF A FUEL-BINDER CONSISTING ESSENTIALLY OF A THERMOSETTING GLYCIDLY ETHER MIXTURE OF (1) AN AROMATIC DIGLYCIDYL ETHER HAVING THE FORMULA 